This invention relates to a miniature electrostatic actuation device capable of generating movements in a determined direction, these movements being obtained through at least one pair of electrodes, one of which can be moved with respect to the other, under the effect of an electrostatic force exerted between the two electrodes subjected to a potential difference.
The invention also relates to an installation comprising such miniature electrostatic actuation devices, the devices being placed adjacent to each other.
For example, the invention relates to a MEMS (Micro Electro Mechanical System) type component in domains such as astrophysics or ophthalmology, particularly in order to actuate optical instruments such as continuously deformable micro-mirrors.
Moreover, the invention is equally applicable to any other micro-system that needs to be actuated in translation along a determined direction.
Several implementations have already been proposed in this technical field.
Firstly, prior art is familiar with conventional miniature electrostatic actuation devices comprising a fixed electrode and a mobile electrode located parallel to the fixed electrode and at a distance from it.
When an electric voltage is applied between the two electrodes, an electrostatic force is generated causing displacement of the mobile electrode with respect to the fixed electrode, along a direction approximately perpendicular to the plane in which the fixed electrode is located. Thus, when such a device is being used, the resulting displacement is capable of generating translation movements along the determined direction, in this case corresponding to the displacement direction of the mobile electrode.
However, this type of conventional actuation device has many disadvantages that strongly disturb its operation. In particular, these disadvantages occur when such a device is being used to continuously actuate a deformable micro-mirror.
These disadvantages include the sudden pull-in phenomenon applied to this type of device. Conventional electrostatic devices normally have a characteristic instability that results in sudden pull-in of the mobile electrode in contact with the fixed electrode when the voltage applied between the two electrodes exceeds a given value. Consequently, the controlled movement distance of the mobile electrode corresponds to a distance that is very much restricted from the initial distance separating the two electrodes to which a potential difference has not yet been applied. The result is that conventional devices are incapable of generating controlled high amplitude movements in the given direction.
Another disadvantage related to this type of device relates to the lack of linearity between the applied voltage and the generated displacement of the mobile electrode. This constraint occurs throughout the usage range and makes it very difficult to control the device, obviously to the detriment of the actuation precision of the micro-system to be controlled.
Finally, it should be noted that the force developed by movement of the mobile electrode remains relatively small, particularly with respect to the average force required to correctly actuate a continuously deformable micro-mirror.
Prior art proposed another miniature electrostatic actuation device described in document EP-A-0 592 469 in order to overcome these various problems.
The document mentioned above describes a miniature actuation device comprising a diaphragm type membrane and a support on which an electrically conducting layer is supported, together with an insulating layer inserted between the diaphragm and the conducting layer. In the inactive state, in other words when no electrical voltage is applied between the electrically conducting diaphragm and the conducting layer, only the ends of the diaphragm are pulled in to come into contact with the conducting layer. The other part of the diaphragm is then at a distance from the conducting layer, so as to define an empty space with it.
When the device is actuated by the application of a voltage between the two electrically conducting elements, the part of the diaphragm at a distance from the conducting layer might progressively get pulled in to come into contact with the conducting layer, symmetrically about a central part of the diaphragm and from its ends. It should be noted the symmetry of the device introduces a translation displacement of the central part of the diaphragm along a direction approximately perpendicular to the conducting layer.
The actuation device described above is advantageous to the extent that the diaphragm used may be gradually pulled into contact with the conducting layer so as to make the volume of the empty space almost zero, without introducing the xe2x80x9cpull-inxe2x80x9d type phenomenon causing the diaphragm to come into sudden contact with the conducting layer. In this way, the controlled stroke of the central part of the diaphragm that may be moved along a determined direction corresponds approximately to the initial distance between the central part of the diaphragm and the conducting layer. This stroke is thus fully optimised as a function of the design of such a device.
Furthermore, experiments carried out on this device have demonstrated the existence of linearity between displacement of the central part of the diaphragm and the applied voltage between the conducting elements, this linear relation having been observed over a wide usage range of the device.
Furthermore, as mentioned above, the diaphragm may gradually be pulled into contact with the conducting layer from the ends of this diaphragm, the contact surface between the diaphragm and the conducting layer depending on the electrical voltage applied between these two elements. As a result, it can be seen that a portion of the diaphragm that is not yet in contact and that is directly along the extension of a part that is in contact is extremely close to the conducting layer, at all times while the device is being activated. The force developed by displacement of the diaphragm is inversely proportional to the square of the distance separating the diaphragm and the conducting layer, therefore this force is very high within the usage range of the device.
Note that the progressive pull-in phenomenon between two conducting elements, known particularly under the term xe2x80x9czippingxe2x80x9d, is also described in the document entitled xe2x80x9cA New Electrostatic Actuator providing improved Stroke length and force, MEMS"" 92, J. Branebjerg; P. Gravesenxe2x80x9d.
Although the miniature device described in document EP-A-0 592 469 solves problems related to conventional electrostatic actuation devices, it does have a major disadvantage.
After an activation phase of such a device that caused pull-in of the diaphragm into contact with the conducting layer, a simple action to reduce the voltage applied between these two elements is not sufficient to put the diaphragm back into its initial position. Thus, in order to overcome this disadvantage, the device also comprises means for injecting gas under pressure inside the empty space formed between the diaphragm and the conducting layer. In this way, when gas is injected under pressure into the empty space, a force is applied over the entire diaphragm so that the diaphragm can resume its initial shape and position.
Nevertheless, this type of arrangement very much complicates the design of this type of device, such that costs related to the manufacture and use of pressurised gas injection means are excessive compared with the total cost of the overall device.
Furthermore, due to the operating mode of the means necessary to put the diaphragm back into its initial position, the diaphragm and/or the insulating layer must necessarily be sealed in order to assure good cooperation with the means of injecting the pressurised gas, thus naturally introducing constraints in the geometry and in the choice for materials that can be used to make the diaphragm and/or the insulating layer of the device.
Therefore, the initial object of the invention is to propose a miniature electrostatic actuation device capable of generating movements along a determined direction, the device comprising at least one pair of electrodes separated by an insulating layer, one of which is mobile, the device at least partially correcting the disadvantages mentioned above related to embodiments in prior art.
More precisely, the object of the invention is to present a device with a simplified design and at lower cost than actuation devices known according to prior art.
Another object of the invention is to propose an installation comprising several miniature electrostatic actuation devices, like those that also satisfy the object of the invention mentioned above.
To achieve this, the invention is related to a miniature electrostatic actuation device capable of generating movements in a given direction, the device comprising at least one pair of electrodes separated by an insulating layer, each pair of electrodes being composed of a fixed electrode attached to a device support and a mobile electrode, at least one portion of which is located at a distance from the fixed electrode when the device is in an inactive state, the device being capable of being activated by means of an electrical voltage applied between the two electrodes in each pair causing the mobile electrode to be pulled into contact with the fixed electrode on a variable pull-in surface that varies as a function of the voltage applied between the two electrodes. According to the invention, the device also comprises an actuation element connected to the portion of each mobile electrode located at a distance from the associated fixed electrode, the actuation element capable of occupying a rest position when the device is in an inactive state, and able of being guided along the determined direction when the voltage applied between the two electrodes in each pair varies, the device also comprising at least one return arm capable of pulling the actuation element back towards its rest position when the voltage applied between the two electrodes in each pair of electrodes is reduced.
The miniature actuation element according to the invention has all the advantages related to the zipping effect used between each pair of electrodes in the device, namely quasi-linearity between displacement of the actuation element and the voltage applied between the electrodes in each pair of electrodes, the long stroke of the actuation element, and the large force developed during movements of this actuation element.
Furthermore, the device also comprises simple mechanical means of putting the actuation element back into place in its rest position following a device actuation phase. These means consist at least of a return arm capable of generating a force opposing the electrostatic force created between the electrodes in each pair of electrodes. The result of this is that it is easily possible to design a device in which the return arm(s) generate(s) a force exceeding the electrostatic force generated when the applied voltage drops below a predetermined value, this consequently causing the actuation element to automatically return to its rest position.
Furthermore, when the voltage applied between the electrodes in each pair of electrodes is simply reduced, the generated electrostatic force reduces such that the return arm(s) provided is (are) capable of bringing the actuation element back towards its rest position, the return movement stopping when the generated electrostatic force exceeds the force created by each return arm.
Preferably, each return arm can work in tension or in bending, and is carried by a support element for each return arm, the return arm being fixed to the support of the device.
Furthermore, the device may be designed such that when the actuation element is in its rest position, the entire mobile electrode of each pair of electrodes is kept at a distance from the fixed electrode by each return arm, the mobile electrode comprising a first end composed of the portion connected to the actuation element, and a second end composed of a priming area that may be pulled into contact with the fixed electrode when a priming voltage is applied between the two electrodes. Thus, in this configuration, the mobile electrodes may be made approximately plane, and consequently there is no longer any need to introduce complex three-dimensional shapes to ensure that there is a part in contact with the fixed electrode and a part kept at a distance from the fixed electrode when the device is not activated, as was the case in prior art. Therefore it becomes much easier to make the mobile electrodes, for example they can be made by photolitho-engraving, or by laser cutout in a thin plate of electrically conducting material.
For each pair of electrodes, it is also possible to arrange for the mobile electrode to include an arm that is curved in a plane approximately orthogonal to the determined direction of displacement of the actuation element, the curved arm being connected firstly to the actuation element of the device, and secondly to the priming area of the mobile electrode.
Advantageously, the curvature formed on the mobile electrodes enables the miniature actuation element to be very compact, while enabling the actuation element to move along a very long stroke.
According to a first preferred embodiment of this invention, the actuation element can be guided along the determined displacement direction using means providing translation guidance of the actuation element coupled to the device. It should be noted that these means are provided since the movement transmitted to the actuation element by any one of the mobile electrodes that is pulled in, follows a trajectory that is not linear. Therefore, the purpose of this guide means is to transform the complex movement created at the end of the mobile electrode into a translation movement along the determined direction.
Preferably, the actuation element may be guided along the determined displacement direction through several electrode pairs comprising a common fixed electrode with an approximately plane shape and mobile electrodes arranged symmetrically with respect to the actuation element, the determined direction of displacement of the actuation element then being approximately orthogonal to the fixed electrode.
Due to the symmetry between the different mobile electrodes connected to the actuation element of the device, the movements transmitted to the device are partially compensated to introduce a movement along a determined single direction approximately orthogonal to the fixed electrode.
The actuation element may comprise four pairs of electrodes in which the mobile electrodes are identical and are distributed at 90xc2x0 around the actuation element, to be inscribed in a hollowed out space, with a square cross section formed in the support element of each return arm of the device.
According to a second preferred embodiment of this invention, the mobile electrodes have a disk shaped priming area, and an arm curved in the shape of a circular ring extending approximately all the way around the priming area.
According to a third preferred embodiment of this invention, the mobile electrodes have a rectangular shaped priming area and a curved L-shaped arm.
According to a fourth preferred embodiment of this invention, the device comprises three pairs of electrodes, in which the mobile electrodes are identical and are designed to be inscribed within a hollowed out space with a hexagonal section formed in the support element of each return arm of the device. The mobile electrodes are then preferably within a priming area extending over a sector covering about 120xc2x0 around the edge of an internal wall in the hollowed out space with a hexagonal section, the priming area being prolonged by a spiral curved arm.
Preferably, for each of the preferred embodiments described above, each return arm is also fixed to one of the elements among the group composed of the actuation element and the portion of the mobile electrode connected to the actuation element of each pair of electrodes.
Furthermore, it is possible to arrange that the support element of each return arm, the actuation element, the mobile electrode in each pair of electrodes and each return arm of the device are made in a single piece, from an electrically conducting material.
Finally, for each pair of electrodes, it is possible for the actuation element and the mobile electrode to be connected by means of an elastic connection in order to absorb parasite displacements parallel to the fixed electrode, between the mobile electrode and the actuation element of the device.
Another purpose of the invention is an installation comprising several miniature electrostatic actuation elements like those described above and also the subject of the invention, the devices being adjacent to each other and provided with a common support.
Other advantages and characteristics of the invention will become clearer in the detailed non-limitative description given below.